Acoustic ink printing is a promising direct marking technology. It potentially is an attractive alternative to ink jet printing because it has the important advantage of obviating the need for the nozzles and small ejection orifices that have caused many of the reliability and picture element (i.e., "pixel") placement accuracy problems which conventional drop on demand and continuous stream ink jet printers have experienced.
Acoustic ink printers of the type to which this invention pertains characteristically include one or more droplet ejectors for launching respective converging acoustic beams into a pool of liquid ink, typically so that the principal or chief ray of each beam is at a near normal angle of incidence with respect to the free surface of the ink, with the angular convergence of each beam being selected so that it comes to focus essentially on the free ink surface. Printing usually is performed by modulating the radiation pressure each beam exerts against the free ink surface. This modulation enables the effective pressure of each beam to make brief, controlled excursions to a sufficiently high pressure level for overcoming the restraining force of surface tension by an adequate margin to eject individual droplets of ink from the free ink surface on command at a sufficient velocity to cause the droplets to deposit in an image configuration on a nearby recording medium.
Prior work has demonstrated that acoustic ink printers having droplet ejectors composed of acoustically illuminated spherical focusing lenses can print precisely positioned pixels at a sufficient resolution for high quality printing of relatively complex images. See, for example, commonly assigned U.S. patent applications of Elrod et al, which were filed on Dec. 19, 1986 under Ser. Nos. 944,490, 944,698 now U.S. Pat. No. 4,751,530 and 944,701 on "Microlenses for Acoustic Printing", "Acoustic Lens Array for Ink Printing", and "Sparse Arrays for Acoustic Printing", respectively. It also has been shown that provision can be made in such printers for dynamically varying the size of the pixels they are printing, thereby facilitating, for example, the printing of variable gray level images. See another commonly assigned U.S. patent application of Elrod et al., which was filed on Dec. 19, 1986 on "Variable Spot Size Acoustic Printing."
Although acoustic lenses currently are a favored focusing mechanism for the droplet ejectors of acoustic ink printers, it is to be understood that there are known alternatives; including (1) piezoelectric shell transducers, such as described in Lovelady et al U.S. Pat. No. 4,308,547, which issued Dec. 29, 1981 on a "Liquid Drop Emitter," and (2) planar piezoelectric transducers having concentric interdigitated electrodes (IDT's), such as described in a copending and commonly assigned Quate et al U.S. patent application, which was filed Jan. 5, 1987 under Ser. No. 946,682 on "Nozzleless Liquid Droplet Ejectors" as a continuation of application Ser. No. 776,291 filed Sep. 16, 1985 (now abandoned). Furthermore, it will be apparent that the existing droplet ejector technology is sufficient for designing various printhead configurations, including (1) single ejector embodiments for raster scan printing, (2) matrix configured ejector arrays for matrix printing, and (3) several different types of pagewidth ejector arrays, ranging from (i) single row, sparse arrays for hybrid forms of parallel/serial printing to (ii) multiple row, staggered arrays with individual ejectors for each of the pixel positions or addresses within a pagewidth image field (i.e., single ejector/pixel/line) for ordinary line printing. As will be appreciated, practical considerations can influence or even govern the choice of droplet ejectors for some printhead configurations, so the above-identified patent applications are hereby incorporated by reference to supplement this general overview.
Preferably, the size droplets of ink that are ejected by an acoustic ink printer, as well as the velocity at which they are ejected, are substantially unaffected by minor variations in the free ink surface level of the printer, such as may be caused by the gradual depletion and/or evaporation of the ink. Relatively straightforward provision may be made to compensate for readily detected changes in the level of the free ink surface, but it is technically difficult and more costly to detect small surface level changes with the precision that is required to compensate for them effectively. Accordingly, the tolerance of acoustic ink printers to slight changes in their free ink surface levels is an important consideration.
Unfortunately, prior acoustic ink printers have been overly sensitive to variations in their free ink surface levels. For example, spherical acoustic focusing lenses having a usable depth of focus on the order of one wavelength of the acoustic radiation in the ink have been developed for such printers. However, it has been found that variations of only one quarter wavelength or even less in the free ink surface levels of printers embodying these lenses tend to materially affect the size of the droplets that are ejected and the velocity at which they are ejected. Research indicates that the half wave resonances which are created because of acoustic reflections within the resonant cavity or cavities of these printers are a principal cause of this problem.
As will be understood, most of the incident acoustic radiation generally is reflected from the free ink surface of an acoustic ink printer because the ink/air interface inherently is acoustically mismatched. Moreover, the ink necessarily is contained within a finite acoustic cavity, so a significant portion of the reflected radiation tends to be returned to the ink surface after being reflected either from the droplet ejector/ink interface or from an acoustically mismatched interface at the rear of the droplet ejector, depending upon whether the droplet ejector is acoustically matched to the ink or not. Typically, the roundtrip propagation time for the return of the reflected radiation to the free ink surface is shorter than the duration of the very narrow band (i.e., single frequency) rf tone bursts that have been proposed for driving the droplet ejectors of prior acoustic ink printers, so the reflected and the non-reflected radiation that are incident on the free ink surface coherently interfere. This interference may be constructive, destructive, or partially constructive and partially destructive, but the free ink surface levels at which resonant constructive interference and anti-resonant destructive interference occur differ from each other by only one quarter of the wavelength of the acoustic radiation in the ink. Consequently, variations as small as one quarter wavelength or even less in the free ink surface level can significantly alter the effective radiation pressure of the focused beam or beams, unless suitable provision is made to prevent or suppress those resonances.